//步驟一. 簽章原始字串 = 廠商代號 + 時間戳 + 請求的body內容
                string pno = "TMSA000XXX";//廠商代號
                string timestamp = ((int)(DateTime.Now.ToLocalTime() - new DateTime(1970, 1, 1, 0, 0, 0, 0)).TotalSeconds).ToString();//時間戳
                string bodyData = "{ \"ProNum\":\"TMSA000XXX\",\"MessageType\":\"C0401\",\"Main\":{.....} }";//請求的body內容
                string signSource = string.Format("{0}{1}{2}", pno, timestamp, bodyData);
                string apiSignKey = "278A0AA4-1E95-45E3-8FE7-6E6E5834757A";//加密金鑰  請根據平台給予的填入
            //步驟二.使用平台給予的金鑰進行加密(使用HMACSHA512),將加密結果轉大寫
                string signEncrypt = HMACSHA512_String(signSource, apiSignKey).ToUpper();
                    
            //加密方法
                public static string HMACSHA512_String(string input, string secretKey)
                {
                    byte[] bKeys = Encoding.ASCII.GetBytes(secretKey);
                    HMACSHA512 msha1 = new HMACSHA512(bKeys);
                    var hash = msha1.ComputeHash(Encoding.UTF8.GetBytes(input));
                    return string.Join("", hash.Select(b => b.ToString("x2")).ToArray());
                }
                
            

            
                string apiUrl = "http://tms-api.ktnet.com.tw/Order/Add";//範例網址
                HttpWebRequest hwrq = WebRequest.Create(apiUrl) as HttpWebRequest;;
                hwrq.Method = "POST";
                hwrq.ContentType = "application/json";
                hwrq.Headers.Add("api-pno", pno);//廠商代號
                hwrq.Headers.Add("api-Timestamp", timestamp);//時間戳
                hwrq.Headers.Add("api-ClientSign", signEncrypt);//簽章加密字串

            //設置請求bodyData的內容
                byte[] bodyBytes = Encoding.UTF8.GetBytes(bodyData);
                hwrq.ContentLength = bodyBytes.Length;
                Stream poststream = hwrq.GetRequestStream();
                poststream.Write(bodyBytes, 0, bodyBytes.Length);
                poststream.Close();

                using (hwrp = hwrq.GetResponse() as HttpWebResponse)
                {
                    Stream stream = hwrp.GetResponseStream();
                    StreamReader streamreader = new StreamReader(stream, Encoding.UTF8);
                    string ResponseString = streamreader.ReadToEnd();//接收回傳結果
                    streamreader.Close();
                    stream.Close();
                    hwrp.Close();
                }
                hwrq.Abort();
                
            

               
                //可以到 http://phptester.net/ 測試
                <?php
                    echo hash_hmac('sha512', 'TMSDEMOWEB1509379717{"ProNum":"TMSDEMOWEB","Main":{"ODATE":"2017-10-27","OCOD4":"oc146"
                                                                  ,"OCCOD":"0","OSCOD":"","OBAK1":"請中午前送貨","OBAK2":"","OBAK3":""
                                                                  ,"OADD1":"Taiwan965932統一超商飛鳳門市 新竹縣芎林鄉富林路二段556號"
                                                                  ,"OADD2":"","OWAY":"oc146","OPAY":"oc綠界整合金流","OTONO":"99"
                                                                  ,"OTax":9,"ReceiveName":"Peiyao","ReceiveTEL":""
                                                                  ,"ReceiveMobile":"0927504321","ReceiveZIPCode":""
                                                                  ,"ReceiveAddr":"Taiwan965932統一超商飛鳳門市 新竹縣芎林鄉富林路二段556號"
                                                                  ,"ReceiveCTCOD":"","ReceiveComName":"","UseInvoice":"0"}
                                                                  ,"Details":[{"ICODE":"509001","INAME":"NIOU AMAZE 珍白淨肌潔顏慕絲"
                                                                              ,"Unit":"","Qty":"2","Price":"980.0000","TaxBA":"A"
                                                                              ,"Stock":"A001","Remark":""}
                                                                             ,{"ICODE":"508013","INAME":"JUST MOMENT 奇肌秒殺經典必敗組"
                                                                              ,"Unit":"","Qty":"2","Price":"5280.0000","TaxBA":"A"
                                                                              ,"Stock":"A001","Remark":""}
                                                                             ]}'
                                                                  , '66E10461-249A-4A26-B846-180B2F4E3839');
                   結果
                   b77b9769757bb26d0031aac5b2b31957ef2fcced87108a853ea4fde89ae3364262ea7f5e80989b66f8e2cc5b6df5d18825f840d2ea906fd86cd9956e4d886a8b